webmap-import: Remove option --mvtdir-tmp.

Having a shared temporary directory, flock(2)'ed to avoid races, is a
great idea in theory but unfortunately doesn't work so well with
systemd.exec(5)'s ReadWritePaths settings since

    ReadWritePaths=/var/www/webmap/tiles
    ReadWritePaths=/var/www/webmap/tiles.tmp

creates multiple mount points pointing at the same file system and
rename(2)/renameat2(2) can't cope with that.  Quoting the manual:

    EXDEV  oldpath and newpath are not on the same mounted filesystem.
           (Linux permits a filesystem to be mounted at multiple points,
           but rename() does not work across different mount points, even
           if the same filesystem is mounted on both.)

So the options are to either use a single ReadWritePaths=/var/www/webmap,
or --mvtdir-tmp=/var/www/webmap/tiles/.tmp.  Both kind of defeat the
point (we'd in fact want to use --mvtdir-tmp=/var/tmp/webmap/tiles), so
we use mkdtemp(3) instead.

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